Water based silicone is what is normally used under the hood as well as in other places. It dries, has a moderate shine that lasts very well, makes next time's cleaning much easier, and does not collect dust.

In years gone by, there wasn't much plastic under the hood unlike today. So, the chosen products and processes have changed accordingly.

And competent detailers don't use a pressure washer. Well, I guess I should say; they shouldn't use them. They can cause far more harm and problems than you can imagine. Besides, most vehicles today that are in for detailing aren't nearly bad enough to warrant that anyway. But then, as with everything automotive, nothing should surprise us. Even blasting the engine with excessive water pressure.